Molecular mechanisms of cardioprotective effects mediated by transplanted cardiac ckit(+) cells through the activation of an inflammatory hypoxia-dependent reparative response

The regenerative effects of cardiac ckit(+) stem cells (ckit(+)CSCs) in acute myocardial infarction (MI) have been studied extensively, but how these cells exert a protective effect on cardiomyocytes is not well known. Growing evidences suggest that in adult stem cells injury triggers inflammatory signaling pathways which control tissue repair and regeneration. Aim of the present study was to determine the mechanisms underlying the cardioprotective effects of ckit(+) CSCs following transplantation in a murine model of MI. Following isolation and in vitro expansion, cardiac ckit(+)CSCs were subjected to normoxic and hypoxic conditions and assessed at different time points. These cells adapted to hypoxia as showed by the activation of HIF-1 alpha and the expression of a number of genes, such as VEGF, GLUT1, EPO, HKII and, importantly, of alarmin receptors, such as RAGE, P2X7R, TLR2 and TLR4. Activation of these receptors determined an NFkB-dependent inflammatory and reparative gene response (IRR). Importantly, hypoxic ckit(+) CSCs increased the secretion of the survival growth factors IGF-1 and HGF. To verify whether activation of the IRR in a hypoxic microenvironment could exert a beneficial effect in vivo, autologous ckit(+)CSCs were transplanted into mouse heart following MI. Interestingly, transplantation of ckit(+)CSCs lowered apoptotic rates and induced autophagy in the peri-infarct area; further, it reduced hypertrophy and fibrosis and, most importantly, improved cardiac function. ckit(+)CSCs are able to adapt to a hypoxic environment and activate an inflammatory and reparative response that could account, at least in part, for a protective effect on stressed cardiomyocytes following transplantation in the infarcted heart.